1. Field of the Invention
The present invention relates to a positive working planographic printing plate precursor, and more particularly to a positive working planographic printing plate precursor for so-called direct platemaking, for making a printing plate directly from a digital signal of, for example, a computer.
2. Description of the Related Art
Planographic printing is a printing method utilizing a printing plate including an oleophilic area which accepts ink and an ink repellent area (a hydrophilic area) which does not accept ink but accepts a wetting water. Currently a photosensitive or presensitized planographic printing plate precursor (PS plate) is widely used in planographic printing.
The PS plate, which comprises a photosensitive layer disposed on a support member, such as an aluminum plate, has been commercialized and widely employed. The photosensitive layer of a non-image area of the PS plate is removed by an imagewise exposure and a development, and the PS plate is used in the printing, utilizing the hydrophilicity of the surface of the support member and the oleophilicity of the photosensitive layer in the image area. In such a printing plate, a high hydrophilic property is required on the surface of the support member, in order to prevent scumming in the non-image area.
Previously, as the hydrophilic support member or the hydrophilic layer to be employed in the planographic printing plate, an anodized aluminum plate, or a silicate processing of such an anodized aluminum plate for further increasing the hydrophilic property, has generally been employed. Moreover, active research of the hydrophilic support member or the hydrophilic layer utilizing such aluminum plate is being performed. For example, a support member which is undercoated with polyvinylsulfonic acid (for example cf. following patent reference 1) and a technique utilizing a polymer having a sulfonic acid group in the undercoat layer for the photosensitive layer (for example cf. following patent reference 2) have been proposed. In addition a technique utilizing polyvinylbenzoic acid as the undercoat material has also been proposed.
Regarding the hydrophilic layer when a flexible support member such as one of PET (polyethylene terephthalate) or cellulose acetate is used instead of a metallic support member such as aluminum, various technologies have been proposed such as a swelling hydrophilic layer composed of a hydrophilic polymer and a hydrophobic polymer (for example cf. following patent reference 3), a PET support member having a microporous surface of hydrophilic crosslinked silicate (for example cf. following patent reference 4) and a hydrophilic layer including a hydrophilic polymer and hardened with hydrolyzed tetraalkyl-orthosilicate (for example cf. following patent reference 5 or 6).
Such hydrophilic layers provide planographic printing plates showing an improved hydrophilicity in comparison with the prior art and capable of providing prints free of scumming at the start of a printing operation, but have such drawbacks as being peeled off during the course of the printing operation or losing the hydrophilicity over time. For these reasons, a planographic printing plate precursor, in which the hydrophilic layer does no peel from the support member and the hydrophilicity of the surface is not lost even under more severe printing conditions and capable of providing a large number of prints without scumming, is desired. Also from a practical standpoint, a further improvement in the hydrophilic property is currently required.
Patent References    1: JP-A No. 7-1853    2: JP-A No. 59-101651    3: JP-A No. 8-292558    4: European Patent No. 709,228    5: JP-A No. 8-272087    6: JP-A No. 8-507727